Primes-in-numbers

Description

Solutions

R

## prime-factors

library(tidyverse)


#' 因数分解
#' @details 先记录所有的因子，再用 table() 统计频数
primeFactors <- function(n) {
    factors <- numeric(0)
    while (n%%2 == 0) {
        n <- n/2
        factors[[length(factors) + 1]] <- 2
    }
    for (i in seq(3, floor(sqrt(n)), 2)) {
        while (n%%i == 0) {
            n <- n/i
            factors[[length(factors) + 1]] <- i
        }
    }
    if (n > 1) {
        # 还剩一个大质数未被整除
        factors[[length(factors) + 1]] <- n
    }

    exponents <- table(factors)
    prime_factors <- names(exponents)

    map2_chr(.x = prime_factors, .y = exponents, .f = function(factor, exponent) {
        if (exponent == 1) {
            str_c("(", factor, ")")
        } else {
            str_c("(", factor, "**", exponent, ")")
        }
    }) %>%
        str_c(collapse = "")
}


library(testthat)
testing <- function(n, expected) {
    actual <- primeFactors(n)
    expect_equal(actual, expected)
}

test_that("tests", {
    testing(7775460, "(2**2)(3**3)(5)(7)(11**2)(17)")
    testing(7919, "(7919)")
})

#> Test passed 😀